The present invention relates to a method and a device for analyzing a sensor device, the sensor device forming an electric resonator in an oscillating circuit energized by an external excitation voltage.
Although applicable to any sensor devices, the present invention as well as the principles on which it is based are explained here with respect to a viscosity sensor device.
Piezoelectric thickness-mode shear transducers made of quartz, for example, have been used for measuring viscosity for some time now (see, for example, S. J. Martin et al., Sens. Act. A 44(1994) pages 209-218). When such a thickness-mode shear transducer is immersed in a viscous liquid, the resonant frequency of its natural oscillation and its attenuation vary as a function of the viscosity and density of the viscous liquid.
FIG. 4 shows an equivalent circuit diagram of a known viscosity sensor having a quartz resonator. R in FIG. 4 denotes the viscosity sensor or xe2x80x9cresonatorxe2x80x9d in general. In the electric equivalent circuit diagram, TA denotes the dry component and FA the liquid component. Dry component TA has a series circuit composed of a capacitor C1, an inductor L1 and a resistor R1. The liquid component has a series circuit of an inductor L2 and a resistor R2. Dry component TA and liquid component FA are bridged by an additional capacitor C0.
In liquid component FA, resistance R2 is proportional to {square root over (xcex7xcfx81)}, where xcex7 is the dynamic viscosity and xcfx81 is the density of the viscous liquid. R2 represents the viscous attenuation by the liquid. L2 produces the frequency shift due to the viscous liquid, which is also proportional to {square root over (xcex7xcfx81)}. In the case of rough resonator surfaces, L2 also contains components generated by xe2x80x9ctrappedxe2x80x9d liquid components in the rough resonator surface. In the case of a known or sufficiently constant density xcfx81, the quartz resonator may therefore be used to determine viscosity xcex7.
According to the publication by S. J. Martin et al. cited above, these variable electric parameters R2 and L2 may be detected by using resonator R as the frequency-determining element in an oscillator. As an alternative, the impedance spectrum may be determined in the vicinity of the resonant frequency (see Lec et al., Proc. IEEE Ultrasonics Symp. (1997) pages 419-422).
FIG. 5 shows such a known analyzer circuit for the known viscosity sensor according to FIG. 4.
A voltage controlled oscillator VCO is used, which supplies resonator R which is immersed in a liquid, namely oil in this case. The output signal of resonator R is mixed with a reference signal REF in a multiplier M.
Finally, the d.c. component of the resulting signal is determined via a low-pass filter TP. The curve of this output signal over the frequency of voltage controlled oscillator VCO is ultimately used to evaluate the oil viscosity.
This evaluation is performed in a computer 100, which also controls voltage controlled oscillator VCO.
One disadvantage of the known approaches described above is that in characterizing highly viscous liquids, resistance R2 increases greatly, so that in the vicinity of the series resonant frequency the impedance of the resonator is determined to a great extent by capacitance C0 and by leakage capacitance CS in parallel to it also. This makes it difficult to determine the relative equivalent parameters by using an oscillator or impedance spectroscopy. One possible expedient is connecting an inductor in parallel to compensate for C0 and CS in the vicinity of the series resonant frequency of the resonator. One disadvantage here is the required balancing plus the fact that leakage capacitance CS varies under some circumstances.
The method according to the above publication by Lec et al. allows at least partial compensation for the effects of C0 and CS, depending on the nature of the reference branch, but it does not yield an output signal corresponding to viscosity, but instead it is used only to determine a characteristic frequency response which is determined by viscosity.
The method according to the present invention and the corresponding device have the advantage over the known approach that the corresponding circuits are also suitable for measurement of highly viscous liquids. The sensor output signal is an easily processable direct voltage as a measure of the viscosity of the liquid.
The present invention is based on the idea that the interfering influence of static resonator capacitance C0 and leakage capacitances CS is eliminated by determining the amplitude of the resistive in-phase component of the resonator current at the series resonant frequency.
According to a preferred embodiment, the current is detected by a measuring shunt connected to ground.
According to another preferred embodiment, the amplitude of the resistive component of the resonator current is determined by multiplying a signal which corresponds to the resonator current by the external excitation voltage and then filtering the result to form an average value.
According to another preferred embodiment, the external excitation voltage is wobbled, and the peak value of the signal corresponding to the average value is retained with a time constant which is greater than the period of the wobble frequency.
According to another preferred embodiment, the external excitation voltage is frequency modulated (e.g., with a small square-wave signal). This produces amplitude fluctuations in the signal corresponding to the average value. The signal corresponding to the average value is used together with the modulation signal to regulate the excitation mid-frequency to the resonant frequency.
According to another preferred embodiment, the sensor device is a viscosity sensor and has a determination device for determining viscoelastic effects based on the output signal of the first low-pass filter and the output signal of the regulating device.
According to another preferred embodiment, at least one of the multiplication devices is implemented in the circuitry by a switched inverter.
According to another preferred embodiment, the detection device for detecting the current in the oscillating circuit is a transimpedance amplifier.